Heavy drinking (defined as 4+/5+ drinks per occasion for women/men) and alcohol use disorder (AUD) are a significant public health problem. Modestly effective pharmacological and psychosocial treatments for AUD exist, yet some heavy drinking (i.e., relapse) is the most common outcome following AUD treatment. Continued development of innovative and efficacious interventions that reduce heavy drinking and specifically target risk factors for heavy drinking is thus clearly warranted. One novel intervention that has considerable promise for reducing heavy drinking is mindfulness-based relapse prevention (MBRP). MBRP is a behavioral intervention for substance use disorder that was designed to target experiences of craving and other risk factors for heavy drinking. Based on the results of numerous studies, MBRP is feasible and efficacious in the treatment of AUD. However the effect sizes of MBRP remain small and many individuals struggle with engaging in the mindfulness practices early in treatment. There is preliminary evidence from our research group that combining a non-invasive form of brain stimulation, transcranial direct current stimulation (tDCS), may improve engagement with mindfulness practices and lead to significant reductions in heavy drinking following treatment. The goal of the proposed study is to examine the efficacy of a mindfulness + tDCS intervention in reducing heavy drinking and impacting hypothesized mechanisms of behavior change among individuals with AUD who are interested in reducing their heavy drinking. In the proposed study, a research team with complementary expertise in AUD treatment, mindfulness-based interventions, brain stimulation, and cognitive neuroscience will combine self-report, behavioral, and neurophysiological data collection via electroencephalography (EEG) to study the psychological and neurophysiological mechanisms of treatment efficacy following a novel, promising intervention that combines brain stimulation with mindfulness training. The mindfulness based intervention in combination with active tDCS is hypothesized to lead to significant reductions in drinks per drinking day after 8 weeks of treatment and these reductions will be maintained up to 2 months following treatment. Further, the effect of active tDCS on drinks per drinking day at the 2 month follow- up will be mediated by greater mindfulness, greater inhibitory control and reductions in craving and negative affect during treatment and at the post-treatment assessment. Approximately 86 individuals meeting criteria for AUD will be randomly assigned to 8 sessions of either MBRP combined with active tDCS (2.0 milliamp current) or MBRP combined with a sham tDCS (0.1 milliamp current) control condition. The proposed study will examine the efficacy (Primary Aim) and psychological and neurophysiological mechanisms of treatment efficacy using behavioral measures and EEG (Secondary Aim). In addition to addressing the question of whether adding active tDCS to MBRP enhances efficacy, it will further examine issues of neurophysiological and behavioral treatment mechanisms to better inform the design of a future large efficacy trial.